The diffusional reaction between titanium and zirconia was carried out isothermally at 1550°C in argon. The distinct reaction layers in the reaction-affected zone between Ti and ZrO 2 were investigated using analytical scanning electron microscopy, analytical transmission electron microscopy, and electron probe microanalyses. In the metal side, there existed five reaction layers in a sequence of α-Ti(O), Ti 2ZrO+α- Ti(O, Zr), Ti 2ZrO+α-Ti(O, Zr)+β′-Ti (O, Zr), α-Ti (O, Zr)+β′-Ti (O, Zr), and β′-Ti (Zr, O) after cooling. In the zirconia side, two reaction layers were found: near the original interface, β′-Ti coexisted with fine spherical c-ZrO 2-x and Chinese-script-like c-ZrO 2-x, which dissolved a significant amount of Y 2O 3 in solid solution; further away from the original interface, the coarsened intergranular α-Zr was excluded from metastable ZrO 2-x, resulting in the lenticular t-ZrO 2-x and ordered c-ZrO 2-x. An attempt was made to determine and propose the microstructural evolution and formation mechanism of the reaction layers between titanium and zirconia isothermally annealed at 1550°C.